I´m currently travelling Central and South America by bus. If I did it again I would definately get a car, probably starting in the states or Mexico.
I have met several people doing it and you get to see so much more. Even met a guy who is travelling from Canada to Costa Rica in a School Bus, he plans to sell it in Costa for twice the price he paid in the states, which will all but pay for his trip!
If you buy something, make sure it has 4WD and preferably get a Jap model, Toyota etc, they are very reliable, much easier to get repaired and easy to sell if you need too. I would go for a Toyoya 2+2 Pickup or similar, lots of room, you can sleep in the back if need be, and give other travellers lifts

I know it costs about $800 US to ship a car from Panama to Ecuador or Colombia if you want to go that route.
I´m probably going to buy something in Ecuador for the second part of my trip to Argentina.
Good Luck
Keith